This invention relates to a technique for monitoring the metallic sheath of an optical cable fiber prior to service on the cable.
Virtually all providers of long-haul communications services, such as ATandT, now employ optical fiber cables to carry traffic associated with such communications services. A typical optical fiber cable for carrying communications traffic includes one or more optical fibers and a metallic member, usually in the form of a metallic sheath that surrounds the fiber. A plastic cover surrounds the sheath to protect the sheath and the fibers from the elements. In the event that the optical fiber cable is buried underground, the metallic sheath carries a locating signal (tone) that facilitates location of the buried cable in the event of a need for repair or replacement.
Installation of an optical fiber cable, whether above or below ground commences by laying the cable itself. Following laying of the cable, regeneration equipment is installed at spaced apart locations along the cable. The regeneration equipment includes one or more amplifiers for boosting the signal to compensate for losses, as well as a signal generator for applying a cable-locating signal to the cable sheath in the event the cable is buried underground. Usually, installation of the regeneration equipment quickly follows laying of the optical fiber cable so that service can be turned up on the cable as rapidly as possible. However, in some instances, installation of the regeneration equipment may lag the laying of the cable by several months or more because of various delays.
During the interval between laying of the cable and installation of the regeneration equipment (including the locating signal generator for an underground cable), no effective way exists to monitor the condition of the cable sheath. Occasionally, damage may occur to the sheath of the fiber optic cable during installation. For example, during the laying of the cable, the plastic covering may suffer scrapes or nicks that expose the sheath to damage, thus creating a sheath fault. Sheath faults reduce the life expectancy of the cable, especially if the sheath carries a large amplitude cable-locating signal. Moreover, the presence of one or more sheath faults may lead to corrosion and increase the degree of cable damage in the event the cable is struck by lightning.
Thus, there is a need for technique for monitoring the sheath of an optical fiber cable during the interval between laying of the cable and installation of regeneration and cable locating mechanisms.
Briefly, the present invention provides a technique for monitoring the metallic member (e.g., a sheath) of a cable that otherwise carries no signals, as would typically occur during the time between laying the cable and connecting regeneration equipment to the cable at various locations therealong. In accordance with a preferred embodiment of the invention, a signal, typically a 48-volt DC signal, is applied to the metallic member at first periodic intervals for a prescribed duration during each interval. The level of the signal applied to metallic member is measured in synchronism with the periodic application of the signal such that each signal measurement is made not longer than the predetermined duration during which the signal is applied. The measurements are communicated to a central facility for evaluation to determine whether the metallic member exhibits a fault conditions. Preferably, the measurements are accumulated for storage and then are relayed to the central facility in batch.